It is the usually that LHC experiments (and HEP in general) secretariat office is the one handling the submission to arXiv and journals per policy (avoid people conflicts and managing it centrally because the papers under the name of the collaboration not individuals. Sometimes papers can also be submitted by author directly but not the physics results papers and papers which is signed off by the whole collaborations (computing and ML stuff mainly).

I would think arXiv can work something out for these cases.

That could readily change, but established large collaborations could also simply be exempted.